FIG. 13 is a drawing for describing a retroreflective material. Retroreflection is defined as a phenomenon where incident light is reflected back in the direction of incidence regardless of entrance angle. A retroreflective material 80 is configuration of a coating 82 of clear synthetic resin containing a multiplicity of fine particles (beads) 81. Incident light 83 incident on the retroreflective material 80 is refracted inside the particles 81, and then reflected after being focused to one point to become reflected light 84 that once more passes through the particles 81 and returns back in an original direction. Therefore, when viewed from the incident direction of the light, a retroreflective material appears to glow, but does not appear to glow when viewed from a direction different than the incident direction of the light. Furthermore, the retroreflective material 80 can be realized using a different configuration such as a prism formed into a three dimensional shape and the like.
While retroreflective material is widely used in, for example, guide signs, clothing (garment), and the like, the properties thereof must be examined to determine product lifetime since retroreflective performance diminishes with use. One characteristic of retroreflective material is a retroreflective coefficient (cd/m2lx) defined by the ratio of a retroreflective luminance (cd/m2) relative to an incident light illuminance (lx). The value of the retroreflective coefficient is used not only in quantifying properties but can also be used as an index for determining product lifetime for retroreflective material. For example, devices and methods for measuring the retroreflective performance of retroreflective material used in road signs, clothing, and the like are described in Patent Documents 1 and 2.